1. Field of the Invention
This invention relates to a process for presulfurizing a desulfurization catalyst used in a hydrodesulfurization unit of petroleum refinery of high molecular hydrocarbons, and to presulfurization promoter used in the process.
In particular, this invention relates to a novel presulfurization promoter used in hydrodesulfurization of metal oxide in desulfurization catalyst, which is necessary at the start of operation when a desulfurization catalyst is charged freshly in a fixed bed.
2. Description of Related Art
In petroleum refinery of high molecular hydrocarbons, hydrodesulfurization is widely used. The usual hydrodesulfurization is effected with a catalyst of oxide or sulfide of metal such as cobalt, nickel, molybedenum or tungsten supported on carriers of alumina or the like. Since hydrodesulfurization and hydrodesulfurization catalyst are well-known and reported in many documents and are not a subject of the present invention, they will be not described here in details. Their short explanation is described in "Chemical Dictionary 5" pages 64 to 65, published by Kyouritsu-Shupan, Oct. 15, 1981.
In the hydrodesulfurization, at the start of operation or when a hydrodesulfurization catalyst is freshly charged or packed in a fixed bed, the hydrodesulfurization catalyst must be subjected to "presulfurization" or preliminary sulfurization treatment before new petroleum refinery cycle starts. The conventional presulfurization technique is classified into so-called "feed sulfurization method" and "chemical spiking method".
In the "feed sulfurization method", sulfur-containing organic compounds contained in a feed stream are hydrocracked at relatively higher temperature (about 260 to about 320.degree. C.) to produce hydrogen sulfide which is then contacted with metal oxide supported on freshly charged catalyst to produce metal sulfides.
In the "chemical spiking method", artificially prepared sulfur-containing organic compound which is rich in sulfur content is added to a feed stream and is then hydrocracked at relatively lower temperature (about 180 to about 260.degree. C.) to produce hydrogen sulfide with which freshly charged catalyst is sulfurized preliminary.
In the "feed sulfurization method", sulfur-containing organic compounds contained in a feed stream are hydrocracked at relatively higher temperature as about 280.degree. C. and react with hydrogen stream to produce hydrogen sulfide, so that undesired unsaturated hydrocarbons are produced. Reaction temperature, however, can not be elevated rapidly, because rapid elevation of temperature results in generation of unsaturated hydrocarbons, which will be a cause of undesirable deposition of cokes. Therefore, reaction temperature must be elevated gradually so that efficiency of the preliminary sulfurization is sacrificed. In fact, this method requires longer time.
In the "chemical spiking method", the reaction time can be reduced remarkably. However, the "chemical spiking method" requires a larger amount of expensive sulfur-containing organic compound and hence is not economical.
Inventors found such a fact that an amount of hydrogen sulfide can be increased remarkably when a compound having mercapto alkylthio group co-exist together with sulfur-containing compounds contained in a feed stream which is a source of hydrogen sulfide for presulfurizing a new catalyst used in hydrodesulfurization.
In the present invention, a compound having mercapto alkylthio group is incorporated in a feed stream in a temperature range where unsaturated hydrocarbons causing trouble of undesired cokes are difficulty produced (about 180 to about 260.degree. C.) to promote hydrocracking of sulfur compounds in the feed stream, so that a larger amount of hydrogen sulfide produced is contacted with metal oxide in the catalyst.